Mutation or loss of the tumor suppressor gene APC predisposes carriers to adenomatous polyposis coli and, ultimately, colon cancer. In humans, a high proportion of breast cancers exhibit loss of heterozygosity of the APC locus. In mice, inherited mutations in APC also predispose to mammary gland tumors. One way of examining the possibility that APC plays a role in tumor suppression in the breast is to generate mammary gland tissue- specific gene knockouts of APC. A derivative of APC, modified by the intronic flanking of exon 14 with specific recombination sites, for targeted mutagenesis by homologous recombination, will be constructed. Successfully manipulated E(mbryonic) S(tem) cells will be used to generate germline-modified mice. Tissue-specific excision of exon 14 of the chromosomal APC gene will be achieved by a specific recombinase, cre, controlled by a breast-specific WAP promoter, introduced by crossing the knockout mouse with a transgenic mouse. Once obtained, studies of female (and male) mice will include analysis of strain-specific differences by introducing the knockout and transgene by into other mouse strains. Breast- specific modifier loci then can he mapped using linkage analysis in a manner similar to that of Dietrich et al.(l993). Pathways involving other tumor suppressors and selected oncogenes can also be studied with these types of transgenics and knockouts, to form the basis of a general resource, and to elucidate a breast-specific pathways to malignancy and to identify some of the genetic factors that define individuals' risk for developing breast cancer.